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Figure 1
(a) 1a, trans-methylstyrene oxide; 1b, trans-stilbene oxide. 2a and 2b are the respective hydrolysis products. (b) Epoxide ring opening following nucleophilic attack. C—O bond breakage is facilitated by acid stabilization of the leaving-group oxyanion and electron donation to the electrophilic C atom. (c) Mechanism of StEH1-catalyzed epoxide hydrolysis. The carboxylate side chain of Asp105 performs nucleophilic attack on one of the oxirane C atoms. The formed oxyanion is stabilized by the phenol groups of Tyr154 and Tyr235. This alkylenzyme intermediate is subsequently hydrolyzed by a base-activated (His300) water. See Fig. 4[link] for the location of these catalytic residues within the active site. The transient presence of the alkylenzyme can be detected from quenching of the intrinsic protein fluorescence, allowing estimation of its rates of formation (k2) and decay (k3). The rate of formation of the tetrahedral intermediate (k′3 in the figure) is expected to be rate-determining for hydrolysis of the alkyl­enzyme, due to the intrinsic instability of this intermediate (k′′3 ≫ k′3). Note that, formally, k3 measures all steps involved from hydrolysis of the alkyl­enzyme to product release; hence, this has been divided into k′3 and k′′3 for clarity.

IUCrJ
Volume 5| Part 3| May 2018| Pages 269-282
ISSN: 2052-2525
https://doi.org/10.1107/S2052252518003573